1. Field of Invention
The invention relates to the field of semiconductors. More particularly, the invention is directed to group III-V nitride semiconductor films usable in blue light emitting devices.
2. Description of Related Art
The light-emitting diode is the basic component for electronic lighting technology. A light-emitting diode is a relatively simple semiconductor device which emits light when an electric current passes through a p-n junction of the light-emitting diode. As shown in FIG. 1, a light-emitting diode 100 includes a p-type 110 semiconductor material adjacent to an n-type 120 semiconductor material, i.e., a p-n junction, characterized by a bandgap energy Eg 130. The bandgap energy 130 is the minimum energy required to excite an electron 160 from a valence band 140 to a conduction band 150, where the electron 160 becomes mobile. Likewise, the bandgap energy 130 also determines the energy of a photon produced when the electron 160 in the conduction band, i.e., a conduction electron, recombines with a hole 170, i.e., an unoccupied electronic state, in the valence band 140. When forward current passes through the diode 100, the electrons 160 in the conduction band 150 flow across the junction from the n-type material 120, while the holes 170 from the valence band 140 flow from the p-type material 120. As a result, a significant number of the electrons 160 and the holes 170 recombine in the p-n junction, emitting light with an energy Ephoton=Eg. These semiconductor devices, comprising a p-n junction, in a single material, and are referred to as homojunction diodes.
In order to obtain more efficient LEDs and laser diodes, in particular, lasers that operate at room temperature, it is necessary to use multiple layers in the semiconductor structure. These devices are called heterojunction or heterostructure LEDs or lasers.
The wavelength, and thus the color of light emitted by an LED or laser diode, depends on the bandgap energy Eg. LEDs or laser diodes that emit light in the red-to-yellow spectrum have been available since the 1970""s. There has been great difficulty, however, in developing efficient LEDs that emit light at shorter wavelengths. Extending LED light sources into the short-wavelength region of the spectrum, the region extending from green to violet, is desirable because LEDs can then be used to produce light in all three primary colors, i.e., red, green, and blue. Shorter-wavelength laser diodes will likewise enable full-color projection displays; and they will also permit the projection of coherent radiation to focus laser light into smaller spots. That is, in the optical diffraction limit, the size of the focused spot is proportional to the wavelength of the light. Reducing the wavelength of the emitted light allows optical information to be stored at higher densities and read out more rapidly.
FIG. 2 shows a conventional LED structure 200 in which an InGaN active layer 230 is formed over a group II-V nitride layer 220. Specifically, as shown in FIG. 2, the conventional LED 200 includes a substrate 205, which may, for example, be formed of sapphire or silicon carbide. A buffer layer 210 is formed on the substrate 205. The group III-V nitride layer 220 is then formed on the buffer layer 210. The group III-V nitride layer 220 is typically GaN. The InGaN active layer 230 is formed on the group III-V nitride layer 220. A second group III-V nitride layer 240 is then formed on the InGaN active layer 230. A third group III-V nitride layer 250 is formed on the second group V layer 240. The first group III-V nitride layer 220 is n-type doped. The second and third group III-V nitride layers 240 and 250 are p-type doped. A p-electrode 260 is formed on the third group III-V nitride layer 250. An n-electrode 270 is formed on the first group III-V nitride layer 220.
This invention provides group III-V nitride films formed on substrates usable to form short-wavelength visible light-emitting optoelectronic devices, including light-emitting diodes (LEDs) and diode lasers.
This invention provides a method for growing light-emitting device heterostructures over a thick InGaN layer that provides a suitable bandgap for blue, green, or even red light emission.
The invention provides a stable InGaN structure that avoids lattice mismatch.
The invention provides other electronic devices, such as transistors, which can incorporate InGaN with other group III-V semiconductors.
Group III-V nitrides include elements from groups III, i.e., gallium, indium, and aluminum, and V, i.e., nitrogen, of the periodic table. These materials are deposited over substrates forming layered structures for optoelectronic devices, including LEDs and laser diodes. The resulting devices can emit visible light over a wide range of wavelengths.
The performance of the optoelectronic devices depends on the quality of the group III-V nitride films formed over the substrates. An important structural characteristic of the group III-V nitride films, which effects their emission quality, is lattice matching between each of the layers. In particular, lattice mismatch occurring between dissimilar materials may produce crystal defects, such as dislocations, cracks, or alloy inhomogeneity, which degrade the optoelectronic quality of the material.
The group III-V nitride semiconductors, GaN, AlN and InN, are used in visible light emitters because these materials are characterized by a wide bandgap energy, as is necessary for short-wavelength visible light emission. Group III-V nitrides also form strong chemical bonds, which make the material very stable and resistant to degradation under high electrical current densities and intense light illumination.
Most optoelectronic devices based on the group III-V nitride compounds require growth of a sequence of layers with different bandgap energies and refractive indices. The bandgap energy of the active layer determines the wavelength of light emitted from a light-emitting diode or laser. In addition, the energy band and refractive index discontinuities between layers of different composition provides for optical and carrier confinement. To obtain layers with the bandgap around 2.7 eV, which will produce light in the blue region of the spectrum, InGaN alloys can be used. The bandgap energy of GaN is 3.4 eV, while the bandgap energy of InN is 1.9 eV. Therefore, InxGa1xe2x88x92xN alloys span the visible spectrum, in which case an estimated In composition x of about 30%, i.e., In0.3Ga0.7N, is required to obtain blue-light emission, 50% for green emission, and 100%, i.e., InN, for red emission.
Growing InGaN alloys with such a high In content on GaN has heretofore been, if not impossible, difficult using conventional techniques, such as metal-organic chemical vapor deposition (MOCVD). Specifically, when using these conventional techniques, the InGaN alloy active region tends to segregate. As the indium content is increased to produce longer-wavelength emission, the InGaN alloy becomes unstable. As a result of this instability, the InGaN alloy separates, or segregates, into In-rich regions and Ga-rich regions, so that the InGaN alloy composition, and therefore the active region bandgap energy, is no longer uniform.
This inhomogeneous composition causes the electroluminescence (EL) to be spectrally broad, i.e., where a broad range of wavelengths is emitted. For instance, while the spectral emission widths of violet LEDs (390-420 nm.), corresponding to 10-20% In content, may be as narrow as 10-15 nm, the spectral emission width increases to 20-30 nm for blue LEDs (430-470 nm.), corresponding to a xcx9c30% In content, and 40-50 nm for green LEDs (500-530 nm.), corresponding to a xcx9c50% In content.
The poor spectral purity of green LEDs limits their application in full-color displays, where pure colors are needed to generate, by additive mixing, a broader palette of colors. Likewise, such broad spectral emission widths also translate into a broad gain spectrum for laser diode structures. When the gain spectrum becomes broad, the peak gain is reduced, so that it becomes difficult to reach the laser oscillation threshold. For this reason, when formed using these conventional techniques, the performance of blue and green group III-V nitride laser diodes is poor compared to violet-emitting group III-V laser diode devices. Indeed, a true blue nitride laser has not yet been demonstrated; and green nitride laser diodes present even greater difficulty, due to their requisite higher In content.
In order to improve the spectral purity of blue and green LEDs, and to promote the development of true blue or green group III-V nitride laser diodes, the growth of high-indium-content InGaN alloys, with homogeneous alloy content, is necessary. The alloy segregation problems must be overcome, so that the alloy content remains uniform, even when the indium content approaches 50%. Presently, alloy segregation limits the In content of nitride lasers to values less than 20%, corresponding to violet and near-ultraviolet emission. While this short wavelength is ideal for optical storage, longer wavelengths, i.e., blue-green-red, are required for applications such as projection displays and undersea communication.
Because the bandgap of GaN is 3.4 eV and the bandgap of InN is 1.9 eV, a group III-V alloy with an In composition of about 30% is required in order to obtain the blue light emission in the conventional LED structure 200 described above. However, GaN and InN have a very large lattice mismatch, which may induce phase separation of InGaN alloys of high In content. Thus, it has heretofore been very difficult to form an InGaN alloy having an In content higher than 20%, where excellent optoelectronic quality is preserved, using conventional growth techniques. Thus, constructing effective pure blue, green, or red light emitting structures using InGaN grown over group III-V nitride layers has proven very difficult.
The inventors have determined that these problems may be caused by the lattice mismatch of over 10% between GaN and InN, which can cause alloy segregation. Thus, InxGa1xe2x88x92xN alloys with homogeneous alloy content x higher than 20% have been difficult to achieve using the conventional techniques, such as MOCVD.
Thus, it would be advantageous to integrate InGaN with other group III-V nitrides, in a manner which avoids the problems of the conventional structures described above.
This invention thus provides a novel semiconductor structure that incorporates a thick InGaN layer. In the modified layer structure according to the invention, a thick InGaN layer replaces the thick GaN layer, which is normally incorporated into the device structure, as both a dislocation filter and a lateral n-contact layer. Depositing a thick InGaN layer establishes a larger lattice parameter compared to the typical GaN template which is employed for overgrowth of the device heterostructure. Consequently, a high-indium-content heterostructure active region grown over the thick InGaN layer experiences less mismatch strain compared to the high-indium-content heterostructure conventionally grown over GaN. Therefore, the device is less likely to suffer structural degradation due to alloy segregation. In this manner, a thick InGaN structure enables the growth of high-indium-content active regions with improved structural and optoelectronic properties. By overcoming the obstacles associated with the InGaN alloy segregation, the compositional uniformity of InGaN layers is improved, so that the spectral emission of blue, green, and even red LEDs becomes more pure. Similarly, the gain spectra of visible nitride laser diodes is also sharper, so that the peak gain is greater, for low threshold. A further benefit of the thick InGaN layer is its superior function as a lateral contact layer. This is a consequence of InGaN""s lower bandgap energy, which contributes to a lower contact resistance, and to higher electron mobility and concentration.
Co-pending U.S. application Ser. No. 09/137,112 filed herewith, is herein incorporated by reference in its entireties.
These and other features and advantages of invention are described in or are apparent from the following detailed description of the preferred embodiments.